Non-contact measurement of a target surface's potential relies on the detection of an electric field. As discussed in D. P. Loconto, High-sensitivity Micromechanical Electrostatic Voltmeter, M. S. Thesis, UC Berkeley EECS, 1992 and R. D. Houston, Non-contacting ESV Technology, Xerox Corporation Internal Report, Rochester, N.Y., March 1983, both of which are hereby incorporated herein by reference in their entireties, the measurement of electric field strength can be made by cutting the electric field lines between the target surface and an on-chip sense-plate conductor. Together, the target surface and the sense plate form a parallel-plate capacitor structure, denoted as the sense capacitance. A third conductor, the shutter, is held at the same potential as the sense plate and swept across the sense-plate. The shutter's motion serves to modulate the effective area of the sense capacitance relative to the target surface. The varying coupling capacitance results in a dynamic current, proportional to the electric field between the target and the sense plate, that is conditioned by on-chip circuitry.
Features of modern MEMS processing, such as Analog Device's iMEMS3 process, allow for the design and manufacture of a high-performance field-mill. Referring to FIG. 2, the MEMS processing provides direct connection between the interface circuit and the key MEMS structures. The direct connection between the circuit and the shutter reduces parasitic capacitance, which improves the signal-to-noise ratio of the device. In addition, co-integration of the MEMS and the circuitry provides high reliability, as demonstrated by the sub-ppm field failure rate of automotive sensors. The two poly layers underneath the sensor provide suitable conducting paths for interconnect and the field-mill sense plate, while the top mechanical poly layer is free to move and provide the mechanical shutter. The composite system architecture is illustrated in FIG. 3.
Non-Contact Voltmeter Measurement Principle: Servo Loop #1
The iMEMS3 field-mill allows for single-chip implementation of a non-contact voltmeter. The simple field mill architecture, however, can suffer from sensitivity to environmental fluctuations. Examples of these potential error sources include gap variations between the MEMS transducer and the target surface, and variations in the absolute shutter motion due to process and temperature variation. To correct for these variations, the field mill may be placed within a global feedback loop to form a true voltmeter, for example, as discussed in K. S. Lion, Instrumentation in Scientific Research, McGraw-Hill, 1959, which is hereby incorporated herein by reference in its entirety. The function of the feedback loop is to adjust the supply reference for the MEMS transducer until it is equal to the unknown target surface potential. When the target, shutter and sense plate are at equal potentials, the motion of the shutter no longer induces a modulation signal and the loop is balanced. The architecture for this instrument is illustrated in FIG. 4. Note that for large target potentials, the bootstrap voltage must be created from a “boost” converter or a regulated high-voltage power supply. The need for a high-voltage bootstrap creates significant system complexity and cost.
Non-Contact Voltmeter Measurement Principle: Servo Loop #2
A low-voltage feedback strategy can be achieved by local charge cancellation of the induced field-mill charge. One basis for this servo strategy is to superimpose an AC feedback signal onto the summing node of the interface amplifier. Once again we start with a modulating field-mill capacitor Cm, which creates a current equal to Vs(dCm/dt) at the summing node (Vs is the potential difference between the target and detector). As illustrated in subfigure 2 in FIG. 5, a time varying signal (Vg) is supplied to the summing node reference, which reacts with the static capacitance (Co) to create a current modulation equal to Co(dVg/dt). Note that this signal is proportional to the nominal gap between the target surface and the detector, just like Cm. If we then close a servo loop by adjusting the AC voltage applied to the summing node (subfigure 3) to cancel the net current, we achieve a net signal at the output of Vout=Vs*(Cm/Co), assuming the frequency and phase of the field-mill and AC signal are matched. This technique relies on the modulation capacitance to be significantly smaller than Co, and U.S. Pat. No. 4,797,620, which is hereby incorporated herein by reference in its entirety, clearly states this can be achieved with gap modulation between the target and the detector. Assuming this is the case, the transfer function for the detector reduces to Vout=(Delta)/go*Vs, where (Delta) is the modulation length, and go is the nominal gap.
This voltmeter architecture suffers from several drawbacks. First, the ratio of (Delta)/go is not really a systematic constant and restricts the accuracy and/or range of the part. Although sufficient spacing between the detector and the target surface can limit sensitivity variations to under 1%, this accuracy comes at the expense of resolution due to noise and susceptibility to external field perturbations. An additional drawback of this architecture is that it is limited to gap modulation in the direction of the target surface. In a MEMS device, significant lateral motion is generally easier to achieve than vertical motion, and this architecture will therefore be difficult to implement effectively. In addition to mechanical constraints, the servo architecture does not lend itself to a lateral implementation for key electrical reasons. Notably, to achieve acceptable attenuation of the high voltage signal, the ratio between the reference capacitance Co and the modulation capacitance Cm must be large. This cannot be achieved with a single lateral plate, where Co and Cm are effectively the same.